1. Field of the Invention
The present invention is in the field of integrated circuits. More particularly, the present invention is in the field of flip-chip packages and methods and apparatus pertaining to planarizing the solder bumps on the surface of an integrated circuit die intended to be part of a flip-chip package.
2. Description of the Related Art
In flip-chip packages, the active face of an integrated circuit die has solder bumps thereon for coupling to a substrate. The die is brought into contact with the substrate such that solder bumps are aligned with solder bump pads on the substrate and the resulting structure is then heated, such that the solder bumps reflow and mechanically couple with the solder bump pads on the substrate.
When an integrated circuit die with its depending array of solder balls is placed on a substrate, which has been prepared with pads of solder paste (solder particles in flux) on the contacts of the circuit board, if the die and substrate are sufficiently planar, then each of the solder balls will contact its corresponding pad of solder paste. Subsequently, when the die and substrate are heated in the solder reflow furnace, the reflowed solder balls and coalescing solder paste will join by mutual surface tension.
However, current solder paste dispense processes result in solder bumps with varying heights. If solder bumps do not contact their corresponding solder paste contact pads, then mutual surface tension is not developed at these locations. When the solder paste and solder balls which are not contacting one another are reflowed, the surface tension of each tends to make them flatten out along their respective surfaces, and actually draw away from one another. Consequently, an electrical connection is generally not effected at the solder balls which do not contact their corresponding pads of solder paste. Also, the solder balls which do not electrically contact their corresponding substrate contact do not participate in the physical securing of the die to the substrate. Moreover, the physical mounting of the die to the substrate is also compromised when solder balls do not join with their corresponding solder paste pads.
One solution to an analogous problem is disclosed in U.S. Pat. No. 5,745,986. This patent discloses a method for planarizing solder balls disposed on the bottom surface of a substrate (as opposed to a die) of a flip chip package. According to this method, a planar surface is brought into contact with the solder balls, which are heated. The force of the planar surface against the heated solder balls results in the planarization of the solder balls.
The technique described in U.S. Pat. No. 5,745,986 is quite effective. However, it would be relatively expensive and time consuming to implement this method in the context of planarizing solder balls on a die surface (as opposed to a substrate surface). Therefore, it would be desirable improve upon this method in the context of planarizing solder balls on a die surface.
The present invention comprises a method of planarizing an array of plastically-deformable electrical contacts on an integrated circuit. According to the present invention, an integrated circuit is placed on a plate with an array of plastically-deformable electrical contacts substantially parallel to and facing the plate, thereby creating an assembly. The integrated circuit is placed above the plate such that the weight of the integrated circuit bears down on the array of plastically-deformable electrical contacts. The assembly is then heated sufficiently to cause individual ones of the plastically-deformable electrical contacts to locally soften but not to cause said individual ones of the electrical contacts to liquefy throughout their volumes. The weight of the integrated circuit applies a force to the softened plastically-deformable electrical contacts, thereby resulting in their planarization.